Hi Woody,
The best use of a topper is to soften up a mattress that is too firm and it will just follow the dips or soft spots in a sagging mattress and any benefit would be partial or temporary at best. You can read more about this in post #4 here.
If you do decide to go with a topper even though it may not be a great solution … then a thicker topper (in the range of 3" or so) would do a better job of making the soft spots and sags more even but it’s not a direction I would suggest…
Some possible or temporary solutions for a sagging mattress that has lost its support are in post #4 here.
Comfort is subjective and different people have different preferences (not everyone prefers latex or foam mattresses and innersprings are still the most popular type of mattress sold in the country) but some of the pros and cons of latex are in this article. In general for those who choose latex it would be a combination of it’s pressure relief, support, and resilient “feel” along with its durability.
When you are looking for a new mattress … the first place I would start is post #1 here which has all the information, steps, and guidelines you will need to make a great choice based on your needs and preferences … regardless of what others may prefer.
Almost all materials including latex come in a wide range of firmness levels so the softness or firmness of latex that someone preferred would also be an individual choice. It seems clear though that soft latex (or any other soft material) is certainly not your first choice although all mattresses need some softness and “give” to relieve pressure.
Phoenix